When drilling an oil well or the like, it is common practice to start with a relatively large diameter hole and cement surface casing in the hole. Subsequent drilling is conducted through this casing. As drilling progresses deeper in the well, the diameter of the hole drilled may be reduced in steps, and progressively smaller diameters of casing may be set in the well at increasing depth. Generally speaking it is desirable to drill as deep as possible with a given hole diameter so that smaller sizes are available for drilling beyond unanticipated problem depths without reducing the hole diameter below a desired size.
A drill bit must pass through the casing in the wall above the depth of drilling. Thus, it is essential that the drill bit have a smaller diameter than the smallest casing already set in the well. It is also important that the hole drilled have a diameter larger than the outside diameter of the next casing to be placed in the well bore. It is desirable to maintain a small clearance between the outside of the casing and the well bore since it is less costly to drill a small hole than a large hole. A small clearance may also permit the use of smaller casing at higher elevations in the hole for a given completion diameter, thereby significantly reducing the total cost of drilling and completing a well.
It is desirable to have as small a difference as possible between the diameters of successive lengths of casing set in the hole. If so, for a given diameter at the completion depth, a larger number of size steps can be provided between the ground surface and the completion depth, or a smaller diameter can be used for the surface casing. Either of these can be of substantial benefit. The use of smaller surface casing is particularly advantageous in undersea well completions. Thus, it is desirable to have a tight fit of casing in a bore hole.
When providing a relatively close fit between a bore hole and casing, it can often be desirable to underream a portion of a well bore to provide a sufficient annulus between the casing and bore hole to inject cement for cementing a portion of the casing in place. There are a variety of other reasons that it is sometimes desirable to underream a well bore.
When a well bore is underreamed, an expandable drilling tool is introduced through the casing to the elevation where underreaming is desired. At that point the underreamer is expanded to drill the rock formation to a larger diameter than the hole through which the underreamer passed. Such underreaming can be in a hole that is already drilled, in which case a bullnose or the like is used as a pilot on the underreamer to maintain it centered in the hole being reamed. In other circumstances the hole is reamed as it is drilled, in which case a rockbit or the like is placed ahead of the underreamer to drill the hole simultaneously with reaming. The rock bit tends to maintain the underreamer centralized in the hole being reamed.
When a short distance is being reamed in a bore hole, a relatively stiff drill string and conventional stabilizers can be employed above the underreamer for assisting in maintaining it centralized in the well bore. When a long distance is being reamed, there has not previously been any apparatus available for centralizing the underreamer from above. Conventional stabilizers are not usable in the underreamed portion of the hole.
Most underreamers have three expandable arms mounted on the underreamer body for supporting the cutters that ream the hole. Geometric constraints limit the increase in diameter that can be reamed with such a tool. It can, therefore, be desirable to employ underreamers with two expandable arms which can have a larger ratio between open and closed diameters.
Such a two-arm underreamer can be made quite stout for obtaining a high penetration rate and prolonged lifetime for underreaming a substantial depth of hole. Such a two-arm underreamer is somewhat more difficult to maintain centralized in a hole, particularly when high drilling weights are applied for maintaining a high penetration rate.
It is therefore desirable to provide an expandable centralizer for use in a drill string above an underreamer. Such an expandable centralizer preferably has a closed diameter approximately the same as that of the underreamer and an open diameter similar to that of the underreamer for maintaining the underreamer centralized in the hole.
The structure of an expandable centralizer preferably provides a high ratio between the expanded and retracted diameters to accommodate underreaming where the underreamer has a high diameter ratio. Preferably the centralizer is positively secured in its open or expanded position to prevent inadvertent collapse during underreaming. It is also desirable that the centralizer be biased toward collapsing to the retracted position when raised in the well bore from the larger diameter portion to the smaller diameter portion to enhance closing of the centralizer.
It is preferable that the centralizer have rollers for engaging the wall of the well so that a high proportion of the torque applied to the drill string is available for underreaming instead of being wasted in friction.
It can also be desirable that similar structure be adaptable to use as a reamer for helping maintain gage in a hole being underreamed, or similar structure may be adaptable for use as an underreamer.